The present invention relates to information storage discs and, more particularly to the design, manufacture, and surveillance of secure storage discs. For the purposes of defining and describing the present invention, it is noted that an information storage disc is a portable, generally planar disc-shaped structure upon which is stored information. The information is commonly stored as computer or machine-readable data on a portion of the disc. For example, by why of illustration and not limitation, two types of common information storage discs are commonly referred to as CDs and DVDs. CDs and DVDs are commonly used to hold information for music, data, or computer software.
The majority of a CD consists of an injection-molded piece of clear polycarbonate plastic. During manufacturing, this plastic is impressed with microscopic bumps arranged as a single, continuous, extremely long spiral track of data. Once the clear piece of polycarbonate is formed, a thin, reflective aluminum layer is sputtered onto the disc, covering the bumps. Then a thin acrylic layer is sprayed over the aluminum to protect it. A label or other graphical image is then printed onto the acrylic. A CD has a single spiral track of data, circling from the inside of the disc to the outside. The data track is approximately 0.5 microns wide, with 1.6 microns separating one track from the next. The elongated bumps that make up the track are each 0.5 microns wide, a minimum of 0.83 microns long, and 125 nanometers high. A laser beam is used to read the data. The laser beam is directed through the polycarbonate layer, reflects off the aluminum layer, and returns to an opto-electronic device that detects changes in the reflected light. The bumps on the disc reflect light differently than the lands (the rest of the aluminum layer), and the opto-electronic sensor detects that change in reflectivity. The electronics in the reader interpret the changes in reflectivity in order to read the data stored on the disc.
A DVD is very similar to a CD, but it has a much larger data capacity. A standard DVD holds about seven times more data than a CD. DVDs are typically of the same diameter and thickness as CDs, and they are made using some of the same materials and manufacturing methods. Like a CD, the data on a DVD is encoded in the form of small pits and bumps in the track of the disc. A DVD is composed of several layers of plastic, totaling about 1.2 millimeters thick. Each layer is created by injection molding polycarbonate plastic. This process forms a disc that has microscopic bumps arranged as a single, continuous and extremely long spiral track of data. Once the clear pieces of polycarbonate are formed, a thin reflective layer is sputtered onto the disc, covering the bumps. Aluminum is used behind the inner layers, but a semi-reflective gold layer is used for the outer layers, allowing the laser to focus through the outer and onto the inner layers. After all of the layers are made, each one is coated with lacquer, squeezed together and cured under infrared light. For single-sided discs, a label is silk-screened onto the nonreadable side. Typically, a mere 740 nanometers separate one data track from the next. The elongated bumps that make up the track are typically each 320 nanometers wide, a minimum of 400 nanometers long, and 120 nanometers high. DVDs can store more data than CDs because of the higher-density data storage in DVDs, the increased storage area on a DVD, and because DVD utilize multi-layer storage.